1. Field of the Invention
The present invention relates to an apparatus for detecting hydroplaning by monitoring the rotation speed of wheels (tires), and, more particularly, to a hydroplaning detector apparatus which properly detects and immediately reports partial hydroplaning (the initial stage of the occurrence of hydroplaning) where a wheel (tire) repeatedly floats above and then contacts the road as water (liquid) penetrates the wedge shape between the road and the wheel (tire), whereby such report enables stable operation of the vehicle.
2. Description of the Related Arts
Hydroplaning means a phenomenon in which, when a vehicle travels over a water film such as a puddle at a high speed (generally 80 km or more, depending on the type of tire), the wheel temporarily floats above the road, and the coefficient of friction suddenly decreases between the road and the wheel. Hydroplaning occurs when a water film penetrates in the wedge shape between the road and the wheel, and its dynamic pressure pushes the wheel upward. Various apparatuses have been proposed which detect the occurrence of hydroplaning and notify the driver of its occurrence to ensure safety driving.
To detect the occurrence of hydroplaning, there is known a technique for utilizing output signals from acceleration sensors which are mounted on the suspension for detecting vertical acceleration (for example, determination through a comparison of the magnitude of output values from the acceleration sensors, and determination based on the level of high-frequency components). There is furthermore known a technique for monitoring output signals from wheel speed sensors.
The technique utilizing the output signals from the wheel speed sensors includes (1) a pattern matching technique comparing typical patterns for changes in the wheel rotation speed when hydroplaning occurs, and determining the occurrence of hydroplaning (Published Unexamined Patent Application No. 5-107257); (2) a speed change differential technique determining the occurrence of hydroplaning based on the gradient of wheel rotation speed change (Published Unexamined Patent Application No. 63-265172); and (3) a technique for determination based on variations in the speed signals from respective wheels (four in total).
The inventor proposed an apparatus in Published Unexamined Patent Application No. 6-11515 which is an apparatus utilizing the pattern-matching technique (1), and which can more properly detect hydroplaning by appropriately modifying a reference change pattern depending on the conditions of vehicle operation, and by appropriately modifying the threshold for determination depending on the conditions of vehicle operation.
However, the various conventional detection techniques were difficult to use in detecting the initial stage of the occurrence of hydroplaning or the partial occurrence of it.
For example, the apparatus utilizing the pattern-matching technique for the results of frequency component analysis proposed in Published Unexamined Patent Application No. 6-11515 can detect the initial stage of occurrence of hydroplaning (partial occurrence of hydroplaning) by modifying the reference pattern depending on the conditions of vehicle operation, but may not always have sufficient detection accuracy.
In addition, the technique using the frequency component analysis can detect the initial stage of the occurrence of hydroplaning (partial occurrence of hydroplaning) by comparing the reference data shown in FIG. 6(a) with the actual data shown in FIG. 6(b) and determining the difference D from the reference data shown FIG. 6(c), but has the following problems:
The first problem is requiring the reference data (reference pattern) shown in FIG. 6(a). Reference data is usually established based on measurement data on a dry road. However, the frequency components in the wheel rotation speed signals vary depending on each of such factors as the type of vehicle, the type of tire to be used, tread patterns, road surface conditions (type of road surface, and degree of unevenness), vehicle speed, load weight, and operating conditions such as direct advance, turning, flat road, uphill, or downhill. Therefore, it is very difficult to select which conditions are to be used as the reference. The detection accuracy is directly affected by the difference between the operating conditions when establishing the reference and the actual run state.
Even if the type of vehicle, the type of tire, and the tread pattern are specified on the assumption of straight operation on a flat road, and reference data is prepared for each factor, the detection accuracy may be degraded because it is substantially not possible to meet changes of the frequency components in the wheel speed data due to variations in pneumatic pressure in the tire or wear in the tread pattern.
If it is intended to maintain predetermined accuracy, the reference data is forced to be changed every time when, for example, the tire is changed. This causes difficulty in maintenance and operation.
As shown in FIG. 6(c), also taken account in calculating the difference D is the level difference in the frequency band for the main frequency components fS centering on a frequency fS proportional to the wheel rotation speed (the absolute value of the difference between the reference level SK of the main frequency components shown in FIG. 6(a) and the level SJ of the main frequency components in the actual run state shown in FIG. 6(b)). However, since the level SJ of the main frequency components fS varies depending on the difference between the type of road surface in establishing the reference data and that in the actual operation, or difference of unevenness of the road surface, it may be unreasonable to directly compared the level SJ with the level SK of the reference data.
Furthermore, the deviation in the center frequency of the main frequency components (the absolute value of the difference between fSK and fSJ) is also utilized in calculating the difference D. However, it is for detecting the fact that hydroplaning occurs in full, and the rotation speed of the wheel becomes no longer proportional to the vehicle speed. Because the vehicle speed maintains the proportional relationship with the frequency fS in the main frequency components for the wheel rotation speed in its initial stage where water starts to penetrate between the wheel and the road in the wedge shape, there arises no frequency deviation .DELTA.fS for the main frequency components fS in the initial stage of hydroplaning.
In its initial stage, as shown in FIG. 7(a), as the degree of penetration of water film between the wheel and the road surface, the area of the residual ground plane of the tire varies, and the resistance from the water film varies so that, in the case of a following wheel, there arises the variation in rotation speed to reduce the rotation speed of the wheel (in the case of a driving wheel, the rotation speed may increase). Thus, as shown in FIGS. 7 (b) and (c), the rotation speed signals may include other new frequency components (particularly, components for frequencies lower than the main frequency), or the level of other frequency components may increase.
Because the above problems are involved, even if the difference D is calculated for the predetermined reference data based on the calculation expression shown in FIG. 6 (c), it was difficult to accurately detect the occurrence of partial hydroplaning.
Furthermore, the conventional determination method prepares different reference data for each of the types of vehicle, the types of tire, and differences in tread patterns making it difficult to use such a method universally for every type of vehicle.